1. Field of the Invention
The present invention relates to monitoring and controlling quality in a continuous sheetmaking machine, and more particularly, to fast machine and cross direction control of headbox and forming elements of a sheetmaking machine using wet end measurements.
2. State of the Art
In the manufacture of paper using a continuous sheetmaking machine, a web of paper is formed from an aqueous suspension of fibers (stock). Stock is dispersed from a dispensing unit referred to as a headbox onto a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dry section where steam heated dryers and hot air completes the drying process. The sheetmaking machine is essentially a de-watering, i.e., water removal system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction. Furthermore, in general, the elements of the system including the headbox, the web, and those sections just before the dryer are referred to as the "wet end". The "dry end" generally includes the sections downstream from the dryer. Papermaking elements and machines are well known in the art and are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking machines are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the art of making paper the sheet properties must be continually monitored and the sheetmaking machine controlled and adjusted to assure sheet quality and to minimize the amount of finished product that is rejected. This control is performed by measuring sheet variables at various stages in the manufacturing process which most often include basis weight, moisture content, and caliper (i.e., thickness) of the sheet, and using this information to adjust various elements within the sheetmaking machine to compensate for variations in the sheetmaking process.
Typically, a scanning sensor is used to perform basis weight measurements of the finished sheet at the dry end of the sheetmaking machine. Scanning sensors are known in the art and are described, for example, in U.S. Pat. Nos. 5,094,535, 4,879,471, 5,315,124, and 5,432,353. The scanning sensor continuously traverses the finished sheet in the cross-direction of the sheetmaking machine. Since the web is moving while the sensor is being scanned, the scanning sensor traverses a diagonal path across the sheet and, as a result, the measured basis weight information provided from the scanning sensor relates to variations in both the machine-direction and cross-direction of the web. The interrelated CD and MD basis weight scanner measurements are further processed and averaged with previous scans to obtain an estimation of independent CD and MD basis weight measurements. Sheetmaking machines are designed with the capability of being independently adjusted to compensate for both CD and MD process variations. The estimated CD and MD basis weight measurements obtained from the scanner are used to control elements in the sheetmaking machine to adjust basis weight in both of these directions.
One of the main disadvantages of scanning sensors is the amount of time that passes from the time that process variations occur in the sheetmaking process to the time the scanning sensor can detect the variations and initiate compensating system adjustments. A typical scan time, (i.e., the amount of time it takes for the scanner to traverse the web) is approximately 16 inches/sec generally resulting in a full sheet scan time of 10-30 seconds. An estimation is obtained by taking 5-8 scans to provide an accurate estimation of the cross and machine direction basis weights. As a result, it can take from 3-15 minutes to obtain CD and MD basis weight measurements using a scanning sensor at the dry end of the sheetmaking machine.
Hence, a sheetmaking machine using a scanning sensor to detect basis weight provides a relatively slow response time to variations in basis weight due to the delay time involved in obtaining basis weight measurements from the scanning sensor. As a result, a sheetmaking machine using a scanning-type sensor is ineffective for detecting rapid variations (i.e., high frequency) in basis weight and particularly variations that occur in the time period less than the amount of time it takes to obtain the basis weight information. In addition, the CD and MD basis weight measurements obtained from the scanning sensor are only an estimation of the actual CD and MD basis weight since the scanning device measurement can only provide interdependent CD/MD basis weight measurements.
What is needed is an manner in which to detect high frequency process variations in an independent manner in both the machine and cross directions and use these detected variations to independently adjust MD and CD controllable elements in the system.